Process for preparing dry solid water absorbing polyacrylate resin

ABSTRACT

A process is disclosed for preparing a solid, water absorbing, cross-linked polyacrylate resin including combining potassium acrylate and a polyvinyl monomer, such as N,N-methylene bisacrylamide, with water in an amount of 55 to 80 combined weight percent of potassium acrylate and polyvinyl monomer based on the total weight of potassium acrylate, polyvinyl monomer and water to form a monomer mixture, and adding a polymerization initiator to the monomer mixture to initiate polymerization of the monomer mixture. The monomer mixture then is polymerized without external heating by utilizing the exothermic heat of reaction as substantially the only non-ambient energy source to drive water away from said polyacrylate resin to form the cross-linked polyacrylate resin having a water content sufficiently low (i.e. 15% or less) to be powdered, such as by pulverization, without an intermediate drying step.

FIELD OF THE INVENTION

The present invention relates to a process for preparing water absorbingdry solid resins.

BACKGROUND OF THE INVENTION

Water absorbing resins have found wide use in sanitary goods, hygenicgoods, water retaining agents, dehydrating agents, sludge coagulants,thickening agents and condensation preventing agents and are also usefulas release control agents for various chemicals.

Water absorbing resins heretofore known include hydrolysis products ofstarch-acrylonitrile graft polymer, carboxymethylcellulose, cross-linkedpolyacrylate products and other resins such as polyvinyl alcohol,polyethylene oxide and polyacrylonitrile resins. Of these waterabsorbing resins, the hydrolysis products of starch and acrylonitrilegraft polymers have comparatively high ability to absorb water butrequire a cumbersome process for production and have the drawback of lowheat resistance and decaying or decomposing easily due to the presenceof starch which is a natural polymeric material. Carboxymethylcellulose,polyethylene oxide and polyacrylonitrile resins still remain to beimproved in water absorbing ability. Among the conventional waterabsorbing resins, polyacrylate cross-linked products appear promising inrespect of both water absorbing ability and stability of quality.

As one of the processes for polymerizing polyacrylic acid or acrylates,an aqueous solution polymerization process is known. The polymerobtained by this process is soluble in water and must therefore becross-linked by some method if it is desired to modify the polymer intoa useful water absorbing resin. However, even if the modification iseffected with use of a cross-linking agent concurrently with or afterthe polymerization by the above process, the resulting reaction productis in the form of a highly viscous aqueous solution or a gel containingabsorbed water which is difficult to handle. Thus the product must bedehydrated (dried) for solidification to obtain a water absorbing resinin the desired form (powder). It is nevertheless difficult to dry thereaction product efficiently by the usual rotary drum roller method orspray drying method because excessive cross-linking frequently takesplace due to overheating during drying or reduced cross-linking densityconversely results from insufficient drying. Extreme difficulties aretherefore encountered in preparing a product of desired water contentand water absorbing ability stably.

SUMMARY OF THE INVENTION

The main object of the present invention is to provide a process forpreparing a water absorbing (cross-linked) acrylate resin product of lowwater content by aqueous solution polymerization without necessitatingany additional dehydrating or drying step.

The other objects and features of the invention will become apparentfrom the following description.

To fulfill the foregoing object, the present invention provides aprocess characterized in that a heated aqueous solution comprisingpotassium acrylate, a water-miscible to water-soluble polyvinyl monomer,water and, when desired, an organic solvent having a boiling point of40° to 150° C., and having a combined monomer concentration of 55 to 80wt. % is subjected to polymerization in the presence of a polymerizationinitiator without external heating while allowing water to evaporateoff.

DETAILED DESCRIPTION OF THE INVENTION

We have conducted extensive research in order to obtain easily andrapidly a dry solid of water absorbing resin having a low water contentand high water absorbing ability by preparing a cross-linkedpolyacrylate product by aqueous solution polymerization whiledehydrating or drying the reaction product with use of the heat evolvedby the polymerization reaction (and cross-linking reaction) withoutnecessitating external heating for the evaporation of water or anyadditional drying step. Consequently we have found that the evaporationof water by the heat of polymerization is greatly influenced by the kindof the monomer salt to be polymerized and also by the combinedconcentration of the monomer salt and the compound used as thecross-linking agent which are contained in the aqueous solution, suchthat when specific monomer salt is used in the form of an aqueoussolution having a concentration adjusted within a specified range, themonomer salt can be polymerized and cross-linked very rapidly withoutexternal heating to achieve a desired degree of polymerization. We havefurther found that water can be evaporated rapidly from the reactionsystem by the heat of the polymerization reaction to readily give thedesired dry solid having a lower water content and high water absorbingability without resorting to any subsequent drying step. We have alsofound that when the aqueous solution further contains an organic solventhaving a boiling point of 40° to 150° C., the temperature of the aqueoussolution is controllable with great ease and the resulting cross-linkedresin has remarkably improved ability to absorb water at an initialrate.

The present invention has been accomplished based on these novelfindings.

When an aqueous solution not containing an organic solvent is to be usedfor the process of the invention, potassium acrylate and awater-miscible to water-soluble polyvinyl monomers are formulated into ahot aqueous solution which contains these monomers in a specifiedcombined concentration. The solution is subjected to a polymerizationreaction (and also to a cross-linking reaction). The polymerizationreaction proceeds sufficiently within a very short period of time whilepermitting the heat of the polymerization reaction to evaporate waterrapidly from the reaction system, whereby a dry solid of water absorbingresin having a low water content can be obtained directly as desiredwithout necessitating any subsequent drying step. The solid can beeasily pulverized into a powder which is suited to the desired use.Moreover, the process of the invention affords, with good stability atall times, a water absorbing resin solid having a very high qualitywithout irregularities in the degree of polymerization which wouldresult from overheating due to external heating or drying step and whichwould lead to impaired water absorbing ability. Further according to theprocess of the invention, the polymerization reaction and evaporation ofwater can be completed very rapidly usually within several minuteswithout external heating. Thus the process can be practiced veryefficiently with greatly reduced consumption of heat energy. The processfurther achieves a high degree of polymerization and is well suited to acommercial operation.

When incorporating an organic solvent according to the invention, theaqueous monomer solution has a solidifying point which is about 10° toabout 20° C. lower than otherwise. This increases the allowable range oftemperature control at least about 3 times. The organic solvent used isvigorously evaporated along with water by the heat of polymerization ofthe monomer. Since the latent heat of the evaporation is considerablysmaller than that of water, the organic solvent functions as a blowingagent in the polymerization reaction system, consequently rendering theresulting resin porous. The resin exhibits about 2 to about 5 timeshigher initial rate of water absorption than the one obtained withoutusing the organic solvent while processing high water absorbing ability.

Thus, the organic solvent, when added to the aqueous monomer solution,produces improved effects without in any way impairing the advantagesresulting from the use of the monomer solution.

According to the process of the invention, a hot aqueous solution isprepared first which comprises potassium acrylate, a water-miscible towater-soluble polyvinyl monomer, water and, when desired, an organicsolvent having a boiling point of 40° to 150° C., and which contains theacrylate monomer and the polyvinyl monomer in a combined concentrationof 55 to 80 wt. %. The concentration of the monomers is deliberatelydetermined considering the state of the solution (i.e. as to whether ornot the monomers can be completely dissolved in water), ease of thereaction of the monomers, escape of the monomers due to the scatteringduring the reaction, etc. The aqueous solution can be prepared easilyusually by placing acrylic acid, potassium hydroxide for neutralizingthe acid and the polyvinyl monomer into water in such amounts that theresulting solution has the above-mentioned monomer concentration. Todissolve the monomers thoroughly, the mixture can be heated at anelevated temperature. According to the present invention, it is criticalto use potassium hydroxide as the neutralizing agent. If ammonia oramine, for example, is used instead, it is difficult to obtain thedesired dry solid of water absorbing resin. Even if a resin is thenobtained, the yield is very low while the resin has low water absorbingability. Further when an alkali metal compound other than potassiumhydroxide, such as sodium hydroxide, lithium hydroxide, cesiumhydroxide, potassium carbonate or sodium carbonate is used as theneutralizing agent, it is difficult to prepare an aqueous solution ofhigh concentration as desired, consequently making it impossible toobtain a dry solid of polyacrylate cross-linked product of low watercontent as contemplated. Although it is desirable to use potassiumhydroxide usually in an amount sufficient to neutralize acrylic acid 100mole %, there is no particular need to neutralize the acid 100% insofaras the hydroxide is used in such an amount as to achieve not less thanabout 70% neutralization. Accordingly the aqueous solution may containup to about 30% of free acrylic acid. However, a large quantity of freeacrylic acid, if present in the aqueous solution, is likely to partlysplash out of the system to result in a loss during the reaction,leading to a reduced degree of polymerization. Use of an excessiveamount of the neutralizing agent will not raise any particular problem,but the excess does not participate in the polymerization reaction andis therefore useless.

The polyvinyl monomer to be used in the invention must be miscible withor soluble in water so that the monomer will be contained in the aqueoussolution of the monomer mixtures as uniformly dissolved or dispersedtherein. Examples of such polyvinyl monomers are usual cross-linkingagents including bisacrylamides such as N,N-methylenebisacrylamide andN,N-methylenebismethacrylamide; polyacrylic (or polymethacrylic) acidesters represented by the following formula (I); and diacrylamidesrepresented by the following formula (II). Among these, especiallypreferably are N,N-methylenebisacrylamide,N,N-methylenebismethacrylamide and like bisacrylamides. Formula (I)##STR1## wherein X is ethylene, propylene, trimethylene, hexamethylene,2-hydroxypropylene, ##STR2## n and m are each an integer of from 5 to40, and k is 1 or 2.

The compounds of the formula (I) are prepared by reacting polyols, suchas ethylene glycol, propylene glycol, trimethylolpropane,1,6-hexanediol, glycerin, pentaerythritol, polyethylene glycol andpolypropylene glycol, with acrylic acid or methacrylic acid. Formula(II): ##STR3## wherein l is 2 or 3.

The compounds of the formula (II) are obtained by reactingpolyalkylenepolyamines, such as diethylenetriamine andtriethylenetetramine, with acrylic acid.

The polyvinyl monomer is used in an amount of about 0.001 to 0.3 wt. %,preferably 0.005 to 0.1 wt. %, of the amount of potassium acrylate inthe aqueous solution to be prepared, or of the combined amount of theacrylate and free acrylic acid, whereby the desired dry solid ofpolyacrylate cross-linked product can be obtained which has high waterabsorbing ability. If a large excess of the monomer is used, theresulting polymer will have exceedingly high strength when in the formof a gel on absorption of water and therefore exhibit impaired waterabsorbing ability, whereas if it is used in too small an amount, apolymer sol will be obtained which conversely has low gel strength toexhibit reduced water absorbing ability, hence objectionable.

According to the invention, it is critical that the aqueous solutioncontain potassium acrylate (and free acyrlic acid) and the polyvinylcompound in a combined concentration of 55 to 80 wt. %, preferably 65 to75 wt. %. The contemplated object can be fulfilled only when the aqueoussolution of such a concentration is used as heated. When the combinedconcentration of the monomers is less than 55 wt. %, it is impossible toobtain the desired dry solid of low water content. The polymer thenobtained can not be made into a powder suited for use unlesssubsequently subjected to a cumbersome drying step which will render theproduct costly and degraded. It is difficult in practice to prepare asolution with a combined concentration of more than 80 wt. % in view ofthe solubility of the monomers.

Examples of organic solvents to be used in the invention when desiredand having a boiling point of 40° to 150° C. are methanol, ethanol,propanol and like alcohol solvents, acetone, methyl ethyl ketone andlike ketone solvents, cyclohexane, n-hexane, n-heptane and likehydrocarbon solvents, benzene, toluene and like aromatic hydrocarbonsolvents, and tetrahydrofuran and like furan solvents. These solventsmay be used singly or in admixture. The solvent is used in an amount of0.5 to 15 wt. %, preferably 1 to 10 wt. %, based on the combined amountof the monomers. With less than 0.5 wt. % of the solvent present, asufficient blowing action will not take place, while the solidifyingpoint of the monomer solution will not lower greatly. Conversely if morethan 15 wt. % of the solvent is used, the resulting resin is likely toexhibit reduced water absorbing ability although achieving a highinitial rate of water absorption. Moreover the monomers are likely toseparate out, hence objectionable. Because the monome solution is heatedprior to polymerization and further because the organic solventevaporates along with water, the boiling point of the solvent is morepreferably in the range of 55° to 120° C.

The aqueous solution of monomers containing the organic solvent admixedtherewith (hereinafter refered to as "mixed solution") is heated andthereafter subjected to a polymerization reaction (and to cross-linkingreaction) with addition of a polymerization initiator. Although theheating temperature is not particularly limited since the mixedsolutiuon is initiated into polymerization by the addition of theinitiator, the temperature is usually about 50° to about 85° C.,preferably about 60° to about 75° C. Various polymerization initiatorsare usable which are known for use in preparing polyacrylates. Examplesof useful initiators are redox initiators comprising a reducing agent,such as a sulfite or bisulfite of an alkali metal, ammonium sulfite orammonium bisulfite, and an initiator, such as a persulfate of an alkalimetal or ammonium persulfate, in combination with the reducing agent;azo initiators including azobis-isobutyronitrile,4-t-butylazo-4'-cyanovaleric acid, 4,4'-azobis(4-cyanovaleric acid) and2,2'-azobis(2-amidinopropane)-hydrochloric acid salt; etc. Theseinitiators can be used singly or in a suitable combination. Of these,especially preferable are a redox initiator composed of ammoniumpersulfate and sodium hydrogensulfite, and azo initiators such aazobisisobutyronitrile and 2,2'-azobis(2-amidinopropane)-hydrochloricacid. These initiators are advantageously used usually in the form of anaqueous solution but can be used as diluted with a suitable solvent. Theinitiator is used in a usual amount, i.e. in an amount, calculated assolids, of about 0.1 to about 10%, preferably about 0.5 to about 5%, ofthe combined weight of the monomers, namely acrylate (and free acrylicacid) and polyvinyl monomer. Depending on the amount and kind of theinitiator, the initiator is usable conjointly with isopropyl alcohol,alkylmercaptan or like usual chain transfer agent to control themolecular weight of the polyacrylate to be obtained.

By the addition of the polymerization initiator, the mixed solution issubjected to polymerization with evaporation of water without heatingthe system from outside. More advantageously the reaction is carried outby admixing a predetermined amount of the initiator or an aqueoussolution thereof with the mixed solution and causing the resultingmixture to flow down onto and spread over a traveling belt. When theinitiator is used in the form of an aqueous solution, care should betaken so that the combined monomer concentration of the mixture will notbe lower than the aforementioned lower limit, i.e. 55 wt. %. Theinitiator can be applied also to the portion of the belt onto which themixture is to be poured.

The polymerization proceeds rapidly after admixing the initiator withthe mixed solution and is completed within a short period of time,usually in about 30 seconds to about 10 minutes. The reaction isexothermic, so that the reaction system is rapidly heated to about 100°to about 130° C. by the heat of polymerization. Consequently waterevaporates from the system rapidly to give a polymer (dry solid) ofdesired low water content without resorting to any external heating. Thewater content of the polymer, although slightly different underdifferent reaction conditions, is usually up to about 15%, preferably upto about 10%. Subsequently the dry solid polymer can be made into thedesired powder easily by a usual method, for example, by pulverizationwithout necessitating any drying step.

The powder thus obtained has outstanding water absorbing ability and isuseful for sanitary goods, paper diaper, disposable duster and likehygenic goods, agricultural or horticultural water retaining agents,industrial dehydrating agents, sludge coagulants, thickening agents,condensation preventing agents for building materials, release controlagents for chemicals and various other applications.

The present invention will be described in greater detail with referenceto the following examples and comparison examples.

EXAMPLES 1

To 22.2 g of deionized water are added 72.1 g of acrylic acid first,then 49.5 g of potassium hydroxide having a purity of 85% and serving asa neutralizing agent, and thereafter 0.01 g ofN,N-methylenebisacrylamide as a polyvinyl monomer to prepare an aqueoussolution of potassium acrylate having a neutralization degree of 75% anda combined monomer concentration of 70 wt. %.

The aqueous solution is maintained at 70° C., and with the solution areadmixed 2.9 g of 18% aqueous solution of ammonium persulfate (0.5 wt. %based on the combined weight of the potassium acrylate, free acrylicacid and N,N-methylenebisacrylamide, the same as hereinafter). Themixture is poured onto a traveling endless belt and spread thereover inthe form of a layer about 10 mm in thickness. About 30 secondsthereafter, the mixture starts to polymerize, and the reaction iscompleted in about 1 minute. The maximum temperature of the mixtureduring the reaction is about 120° C.

The reaction gives a dry solid strip of cross-linked potassiumpolyacrylate product having a water content of 11% and a residualmonomer concentration of 1200 ppm. The strip is made into a powder by apulverizer. The powder has water absorbing ability of 450 as measuredwith use of deionized water or 60 as measured with 1% saline. The waterabsorbing ability is determined by placing 1 g of the powder (ascompletely dried) into test water and measuring the weight (g) of waterabsorbed by the powder.

EXAMPLES 2 TO 9

Polymers are prepared in the same manner as in Example 1 with theexception of varying, as listed in Table 1, at least one of the combinedconcentration of monomers, the temperature at which the aqueous solutionis maintained, the amount of polyvinyl monomer(N,N-methylenebisacrylamide), the kind and amount (degree ofneutralization) of neutralizing agent, and the amounts, based on thecombined amount of the monomers, of ammonium persulfate (APS) and sodiumhydrogensulfite (SHS) serving as the polymerization initiator. Table 1also shows the conditions involved in Example 1.

                                      TABLE 1                                     __________________________________________________________________________         Concn.    Poly-         Neu-                                                  of        vinyl                                                                              Concn. of                                                                              tral-                                                                             Degree of                                    Example                                                                            Monomers                                                                            Temp.                                                                             monomer                                                                            initiator (wt. %)                                                                      ing neutraliza-                                  No.  (wt. %)                                                                             (°C.)                                                                      (wt. %)                                                                            APS SHS  agent                                                                             tion                                         __________________________________________________________________________    1    70    70  0.01 0.5 0.5  KOH 75                                           2    60    70  0.01 0.5 0.5  "   "                                            3    70    75  0.01 0.5 0.5  "   "                                            4    70    70  0.03 0.5 0.5  "   "                                            5    70    70  0.05 0.5 0.5  "   "                                            6    70    70  0.10 0.5 0.5  "   "                                            7    70    70  0.01 0.8 0.8  "   "                                            8    70    70  0.01 1.0 1.0  "   "                                            9    70    70  0.01 0.5 0.5  "   "                                            __________________________________________________________________________

The amount of polyvinyl monomer listed is expressed in % by weight basedon the combined amount of potassium acrylate, free acrylic acid and thepolyvinyl monomer, and the concentration of initiator is expressed in %by weight based on the combined amount by weight (calculated as solids)of the monomers and the initiator, the same as hereinafter.

Table 2 below shows the water content of each dry solid of polymer(cross-linked potasium polyacrylate product) and the water absorbingability of the powder prepared from the solid in the same manner as inExample 1. The water absorbing ability is determined in the same manneras in Example 1.

                  TABLE 2                                                         ______________________________________                                                        Water absorbing ability                                       Example  Water        Deionized 1%                                            No.      Content (%)  Water     Saline                                        ______________________________________                                        1        11           450       60                                            2        15           530       65                                            3         8           500       63                                            4        11           420       57                                            5        13           360       55                                            6        12           280       48                                            7        10           460       55                                            8        11           450       55                                            9        11           420       54                                            ______________________________________                                    

COMPARISON EXAMPLE 1

A polymer salt is prepared in the same manner as in Example 1 exceptthat the combined concentration of monomers is 40 wt. %. The polymersalt has a water content of 42%, cannot be pulverized directly butrequires subsequent drying before pulverization.

COMPARISON EXAMPLE 2

A polymer salt is prepared in the same manner as in Example 1 exceptthat the combined monomer concentration is 50 wt. %. The product has awater content of 33%, cannot be pulverized directly but requiressubsequent heating before pulverization.

COMPARISON EXAMPLE 3

Although it is attempted to prepare an aqueous solution of sodiumacrylate having a combined monomer concentration of 50 wt. %(neutralization degree 100%) in the same manner as in Example 1 with theexception of using sodium hydroxide in place of potassium hydroxide, itis impossible to obtain a uniform solution due to the separation of someprecipitate. The solution does not become uniform even when heated to65° C. An aqueous solution is obtained when it is heated to at least 70°C.

After maintaining the solution at the same temperature, the samepolymerization initiator as used in Example 1 is admixed with thesolution. The same procedure as in Example 1 is thereafter repeated toobtain a polymer salt having a water content of 30%, but the productcannot be pulverized directly and must be dried by heating forpulverization.

COMPARISON EXAMPLES 4 AND 5

Polymers are prepared in the same manner as in Example 1 with theexception of using N,N-methylenebisacrylamide as the polyvinyl monomerin the altered amounts listed in Table 3, which also shows the watercontent and water absorbing ability of each of the polymers obtained.

                  TABLE 3                                                         ______________________________________                                                Amount of         Water Absorbing Ability                                     Polyvinyl                                                                             Water     Deionized 1%                                                Monomer Content   Water     Saline                                    ______________________________________                                        Comp. Ex. 4                                                                             0.0005    13        200     30                                      Comp. Ex. 5                                                                             0.35      12        120     15                                      ______________________________________                                    

On absorption of water, the polymer obtained in Comparison Example 4becomes a sol unlike the polymer obtained by the invention which becomesa gel.

EXAMPLES 10 TO 13

Polymers are prepared in the same manner as in Example 1 with theexception of changing at least one of the amount ofN,N-methylenebisacrylamide and the kind and amount of the polymerizationinitiator as listed in Table 4 below. Table 4 also shows the watercontent and water absorbing ability of each polymer obtained.

                                      TABLE 4                                     __________________________________________________________________________                   Amt. of    Water Absorbing Ability                             Ex.                                                                              Initiator   Polyvinyl                                                                           Water                                                                              Deionized                                                                            1%                                           No.                                                                              Kind    Concn.                                                                            Monomer                                                                             Content                                                                            Water  Saline                                       __________________________________________________________________________    10 2,2'-azobis-                                                                          0.5 0.01  11   520    58                                              (2-amidino-                                                                   propane)hydro-                                                                chloric acid                                                               11 2,2'-azobis-                                                                          0.5 0.02  12   610    65                                              (2-amidino-                                                                   propane)hydro-                                                                chloric acid                                                               12 2,2'-azobis-                                                                          1.0 0.01  10   550    62                                              (2-amidino-                                                                   propane)hydro-                                                                chloric acid                                                               13 2,2'-azobis-                                                                          1.0 0.02  11   580    63                                              (2-amidino-                                                                   propane)hydro-                                                                chloric acid                                                               __________________________________________________________________________

EXAMPLES 14 TO 21

Polymers are prepared in the same manner as in Example 1 except that thecompounds listed in Table 5 below are used as polyvinyl monomers in thelisted amounts. Table 5 also shows the water content and water absorbingability of each polymer obtained.

                  TABLE 5                                                         ______________________________________                                        Ex.  Polyvinyl Monomer Water    Deionized Water                               No.  Kind         Amount   Content                                                                              Absorbing Ability                           ______________________________________                                        14   Ethylene     0.01     12     480                                              glycol diallyl                                                                ester                                                                    15   Ethylene     0.02     13     430                                              glycol diallyl                                                                ester                                                                    16   Diethylenetri-                                                                             0.01     12     510                                              amine-diacryl-                                                                amide                                                                    17   Diethylenetri-                                                                             0.02     12     450                                              amine-diacryl-                                                                amide                                                                    18   N,Nmethylene-                                                                              0.01      9     520                                              bismethyacryl-                                                                amide                                                                    19   N,Nmethylene-                                                                              0.05     11     390                                              bismethacryl-                                                                 amide                                                                    20   Polyethylene 0.01     10     500                                              glycol                                                                        diacrylate*                                                              21   Polyethylene 0.05     11     430                                              glycol                                                                        diacrylate*                                                              ______________________________________                                         *Polyethylene glycol diacrylate used in Examples 20 and 21 is represented     by the following formula:                                                     ##STR4##                                                                 

EXAMPLES 22 TO 25

Acrylic acid (72.1 g), 18.0 g of deionized water, 40.9 g of solidpotassium hydroxide (water content 4%) and 5.2 g of one of the solvents(5 wt. % based on the monomers) listed in Table 6 are mixed together,and the mixture is maintained at 75° C. With the mixture is furtheradmixed 4.0 g of 10% aqueous solution of2,2'-azobis(2-amidinopropane)hydrocholoric acid salt. The resultingmixture is immediately poured onto a traveling endless belt and spreadthereover to a thickness of 5 mm. About 15 seconds later, the mixturestarts to polymerize, and the polymerization is completed in about 30seconds. The maximum temperature of the mixture during the reaction is130° to 135° C.

The reaction gives a dry strip of crosslinked potassium polyacrylateproduct, which is pulverized to a powder 20 to 100 mesh in particlesize.

The same procedure as above is repeated with use of the other solvents.All the powders obtained have a water content of 4 to 6%.

A 0.1 g quantity of each of the powders is accurately measured out andthe water absorbing ability of the powder is measured after immersingthe powder in deionized water or 1% saline for 10 seconds, 30 seconds or15 minutes. Table 6 shows the results.

EXAMPLE 26

An aqueous monomer solution is prepared in the same manner as inexamples 22 to 25 with the exception of not using any organic solventand using 23.2 g of deionized water. The solution is thereaftersubjected to polymerization in the same manner as in these examples toobtain a powder of dry solid. Table 6 also shows the test resultsobtained with this powder.

                  TABLE 6                                                         ______________________________________                                                Example No.                                                                   22     23       24       25    26                                             Organic Solvent                                                       Water                              Tetra-                                     absorbing                          hydro-                                                                              (Water                               ability (times)                                                                         Acetone  Ethanol  Benzene                                                                              furan only)                                ______________________________________                                        1% Saline                                                                     10 Sec.    73       68       70     76    54                                  30 Sec.    83       82       85     85    75                                  15 Min.    97       93       94     96    96                                  Deionized                                                                     water                                                                         10 Sec.   620      690      600    690   300                                  30 Sec.   900      910      880    920   750                                  15 Min.   960      980      900    980   920                                  ______________________________________                                    

EXAMPLES 27 TO 29

Water absorbing resin solids are prepared in the same manner as inExample 22 with the exception of using 3, 5 or 10 wt. %, based on themonomers, of methanol in place of 5.2 g of acetone and varying theamount of deionized water so that the combined amount of the water andthe methanol is 23.2 g. Each of the solids is tested for water contentand also for water absorbing ability by immersion in 1% saline anddeionized water for specified periods of time. The results are given inTable 7, which also shows the results of similar tests conducted withuse of the solid of Example 26.

                  TABLE 7                                                         ______________________________________                                                     Example No.                                                                   27   28        29     26                                         ______________________________________                                        Amount of organic                                                                             3      5        10    0                                       solvent (based                                                                on monomers, %)                                                               Water content of                                                                             7.2    6.0       2.1  10.2                                     resin                                                                         Water absorbing                                                               ability (times)                                                               1% Saline                                                                      5 Sec.        50     58        65   32                                       10 Sec.        64     71        75   54                                       15 Sec.        71     77        79   61                                       20 Sec.        75     80        82   67                                       40 Sec.        85     87        88   80                                       60 Sec.        90     90        91   88                                       15 Min.        97     93        94   96                                       Deionized water                                                                5 Sec.        250    420       570  140                                      10 Sec.        420    650       760  300                                      15 Sec.        590    740       830  410                                      20 Sec.        660    790       850  560                                      40 Sec.        880    900       890  850                                      60 Sec.        920    910       890  900                                      15 Min.        930    910       890  920                                      ______________________________________                                    

What is claimed is:
 1. A process for preparing a solid water absorbingresin comprising mixing an aqueous monomer solution of (A) potassiumacrylate; and (B) a water-miscible or water-soluble polyvinyl monomer ina combined concentration of 55 to 80 wt. %; to form a mixed monomersolution wherein the monomers of the mixed monomer solution consistessentially of (A) and (B) and initiating polymerization of monomers (A)and (B) such that during polymerization, the exothermic heat of reactionis substantially the only heat energy used to accomplish polymerization,cross-linking and to drive off sufficient water to obtain a solidcross-linked resin.
 2. A process as defined in claim 1 wherein thecombined concentration of the monomers (A) and (B) is 65 to 75 wt. %. 3.A process as defined in claim 1 wherein the mixed monomer solution has atemperature of 50° to 85° C. prior to polymerization.
 4. A process asdefined in claim 1 wherein monomer (B) is selected from the groupconsisting of N,N-methylenebisacrylamide andN,N-methylenebismethacrylamide.
 5. A process as defined in claim 1wherein the mixed monomer solution further contains an organic solventhaving a boiling point of 40° to 150° C.
 6. A process as defined inclaim 5 wherein the organic solvent has a boiling point of 55° to 120°C.
 7. The process of claim 1 further including the step of adjusting thetemperature of the mixed monomer solution to a temperature of 50° to 85°C. prior to adding a polymerization initiator to said monomer solution.8. The process of claim 1 wherein monomer (B) is water-miscible.
 9. Theprocess of claim 1 wherein monomer (B) is water-soluble.
 10. The processof claim 1 including adding a polymerization initiator to said mixedmonomer solution in an amount of at least 0.5% by weight of monomers (A)and (B).